wouldnt want to be grinding or drilling fuck all without the facility to flush the entire engine out after tbh. im sure the pickup strainer and oil filter would do their job but its not the point.

id try and split the nut, ideally a proper nut spliter, or a sharp chisel with no plastic or wooden grip and lump hammer. im sure the right sized socket would do the trick belted on though. try imperial sizes, and make sure its a good quality socket. id imagine its 13mm so try beasting a 1/2" on.

try getting one of them aerosol duster things on the cam cap aswell, spray it upside down and freeze the fucker and it might just shrink enough to reduce the force on the nut enough to allow you to undo it with a well fitting socket.